The present invention pertains in general to wellhead expansion assemblies and in particular to wellhead expansion assemblies for use with a plurality of tubes extending both through the wellhead and into a well.
Petroleum within some underground formations is so viscous that the temperature and pressure within the formation are insufficient to cause it to flow to a producing well. Hot fluids, particularly steam, are injected into such formations in order to raise the temperature of the formation and thereby reduce the viscosity of the petroleum contained therein to a point at which the petroleum flows to a producing wellbore.
In some wells, it is desirable to treat more than one horizon with hot fluids. Where these horizons require different injection techniques, which may include the use of fluids at different temperatures and different pressures, separate conduction pathways are used for each different type of fluid.
Commonly, metallic steam injection tubing is run into wells which have been drilled and cased. Packers are placed between the tubing and the casing above and sometimes below the formation to be injected. Next, the wellhead is connected to a source of hot fluid, such as a steam generator. The hot fluid is pumped into the formation through the tubing.
The tubing within the well expands when it is exposed to the hot fluid. Axial expansion forces may cause the tubing to be damaged or buckled between fixed subsurface connections and the wellhead if allowance is not made for the expansion.
One approach to compensating for such thermal expansion involves the use of a dome-type packoff wellhead such as the one disclosed in U.S. Pat. No. 4,401,160, assigned to a common assignee. Where either one or more injection tubings are used according to this approach, the dome-type packoff is secured by means of bolts through flanges welded to expansion chambers. Bolts of the sort required are expensive and cannot readily be unscrewed but must be broken off, necessitating replacement of the bolts whenever a chamber is removed. Where more than one injection tubing is used in a well, all tubing strings beyond a first are provided with an expansion joint between a subsurface packer and the wellhead. A first tubing string is allowed to axially expand through a packing gland in the wellhead and into an expansion chamber.
A disadvantage of this approach for use with more than one tubing string is that the tubing must be removed from the well in order to distinguish between a leaking expansion joint and a leaking downhole packer. This is an expensive procedure. Furthermore, a spline joint within an expansion joint may be stripped during removal so that the downhole packer can no longer be conveniently unscrewed. In addition, because O-rings in expansion joints are prone to leakage, it would be desirable to replace expansion joints with packers, which are less prone to leakage.